Motor vehicles powered by internal combustion engines may waste over 30% of fuel energy due to the venting of exhaust heat to the environment.
To reduce wasted fuel energy, vehicles may recover engine exhaust heat for transfer to various other vehicle systems. For example, US 2009/0241863 describes storing high temperature cooling water in a heat accumulator, which is used to warm up a vehicle engine at engine start. After engine start, exhaust heat is recovered and transferred to cooling water. This heated cooling water is then circulated to the engine to provide further engine warming, and to a heater circuit to heat air supplied to a passenger compartment. In this way, both the engine and the passenger compartment can be warmed up soon after engine start.
However, the inventors herein have recognized that such an approach, where exhaust heat is transferred away from the exhaust system soon after engine start, may increase undesirable exhaust emissions. For example, transferring exhaust heat to cooling water and thus away from the exhaust system may delay catalytic converter “light-off”, which is temperature-dependent. As a result, it may take longer for the catalytic converter to reach a temperature where it can efficiently perform catalytic reduction, thus increasing undesirable exhaust emissions. This delay and associated effect on emissions may be especially pronounced in vehicles with diesel engines, which may output less exhaust heat than other types of engines.
To address the above issues, the inventors herein have identified various approaches for recovering engine exhaust heat for transfer to various other vehicle systems without increasing undesirable exhaust emissions by delaying catalytic converter light-off. In one example approach, heat stored in a phase-change material in a heat battery is delivered to a heat exchanger with a bypass coupled in an exhaust system at engine start-up. After exhaust temperature exceeds a first threshold, heat is delivered from the heat battery to a vehicle cabin and optionally to other vehicle systems requiring heat, and after exhaust temperature exceeds a second, higher threshold, heat is delivered from the heat exchanger to charge the heat battery. For example, the first threshold may be a temperature value greater than or equal to a catalytic converter light-off temperature. In this way, once catalyst light-off is achieved (such that further heating of the catalytic converter is not necessary for emissions reduction), exhaust heat may be transferred via the heat exchanger and delivered to a cabin heating system. Further, the second, higher threshold may be a temperature value at which the amount of available exhaust heat exceeds an amount of heat needed by the cabin heating system and any other vehicle systems requiring heat, such that extra heat is available for charging the heat battery.
In this way, heat stored in the heat battery may be used to expedite catalytic converter light-off after engine start-up, thereby reducing undesirable exhaust emissions, while also being available to improve cabin heating and to provide heat to other vehicle systems requiring heat. For example, heat transfer fluid may circulate heat stored in a phase-change material in the heat battery to the heat exchanger to transfer heat to engine exhaust upstream of a catalytic converter. As the engine warms up, exhaust temperature increases and the catalyst reaches light-off. After catalyst light-off has occurred, exhaust heat may be transferred to heat transfer fluid in the heat exchanger, and the heat transfer fluid may be circulated to a vehicle cabin heating system to heat the vehicle cabin. At this stage, exhaust heat may also be transferred via circulation of heat transfer fluid to one or more other vehicle systems requiring heat, such as the transmission for heating of transmission oil. As the engine warms up further, which causes exhaust temperature to increase further, heat transfer fluid may be circulated from the heat exchanger to a phase-change material in the heat battery, to charge the heat battery. Charging the heat battery in this way enables heat to be stored for delivery to the heat exchanger at a subsequent engine start-up, and can reduce cooling loads for at least a duration. Thus, in accordance with this example method, unwanted exhaust emissions that occur prior to catalytic converter light-off may be reduced. Further, excess exhaust heat may be advantageously utilized rather than vented to the environment as wasted fuel energy.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.